Cooling apparatus

ABSTRACT

Cooling apparatus comprises a cooling system defining a closed path around which a coolant flows. The system includes a pump for causing coolant flow, a supply line extending from the pump to a cold location, positioned in a cryostat, in order to cool that location, and a return line extending from the cold location to the pump. The pump is located externally of the cryostat. A first heat exchanger is positioned within the cryostat and links the supply and return lines to allow heat exchange therebetween such that coolant flowing in the supply line is cooled by coolant flowing in the return line. A refrigerator is provided having a cooling stage within the cryostat and coupled to the supply line downstream of the first heat exchanger such that coolant reaching the first cooling stage has been precooled by the first heat exchanger.

The invention relates to cooling apparatus, for example for use in cooling electrical conductors to a temperature at which they superconduct. The invention is particularly suited for cooling electromagnets to their superconducting condition for use in NMR (nuclear magnetic resonance) and ICR (ion cyclotron resonance) experiments.

High field NMR magnets are often “sub-cooled” to a temperature a few Kelvin below the atmospheric boiling point of liquid ⁴He (4.2K) to improve the critical current capacity of the superconductor and allow a higher magnetic field to be generated. This is commonly achieved using a bath of liquid ⁴He in which the magnet is submerged. The magnet bath or vessel is commonly cooled to ˜2.2K, which is just above the superfluid transition temperature, or λ point, of ⁴He (T_(λ)=2.17K).

2.2K is the preferred operating temperature for two reasons. The specific heat capacity of ⁴He peaks at the λ point (FIG. 2), so it is desirable to operate as near the lambda point as possible to improve the temperature stability of the system. However, it is generally considered undesirable to operate below the λ point. This is because a proportion of the liquid becomes superfluid, with zero viscosity, and it will flow, even against gravity, through the smallest cracks and orifices towards areas of the cryostat at higher temperature, thus causing a large heat leak and increasing boil-off (the so-called “superleak” phenomenon).

In early sub-cooled systems the magnet containing vessel which contained liquid He was simply pumped to a lower pressure, hence gradually evaporating the liquid bath and sub-cooling the magnet. With this simple design it is necessary to warm up the system, and hence de-energise the magnet, when the bath needs re-filling. To avoid this major cost and inconvenience, the lambda point refrigerator was invented by Roubeau and others (“The operation of superconducting magnets at temperatures below 4.2K”, Cryogenics, February 1972, p. 44-47, Biltcliffe, Hanley, McKinnon, Roubeau).

More recently, as shown in FIG. 1, a “lambda point refrigerator” has been used. Referring to FIG. 1 a magnet 2 is submerged in liquid He in a first coolant containing vessel 1 at atmospheric pressure. A second coolant containing vessel 3, which is open to atmosphere, holds a reservoir of liquid He boiling at 4.2K; this reservoir 3 may be refilled at any time. It is connected to the vessel 1 via a quench valve 14. Liquid He is conveyed from the second vessel 3 to a heat exchanger 5 in the first vessel 1 via an (optional) second heat exchanger 6 and an expansion valve 4. The heat exchanger 5 is typically a coiled loop tube immersed in the top of the liquid helium bath of the first vessel 1. The pressure in the loop 5, on the downstream side of the valve 4, is reduced by pumping using an external pump 13, typically to 20-50 mbar. Helium liquid passing through the valve 4 is partially vaporized and cooled by a few Kelvin due to the pressure drop across the valve. The reduced vapour pressure in the loop lowers the boiling temperature of the remaining liquid, which consequently evaporates, absorbing heat from the magnet bath and cooling it via heat exchange through 5. The vapour leaving the heat exchanger 5 is passed through the optional second heat exchanger 6, which pre-cools the liquid entering the valve with the aim of reducing the fraction vaporized in the valve, and hence reducing the mass flow rate required for a given cooling power.

The cooling power of the lambda point refrigerator constituted by components 4,5,6 is given by: $\frac{\mathbb{d}Q}{\mathbb{d}t} = {\left( {1 - \lambda} \right) \cdot \frac{\mathbb{d}m}{\mathbb{d}t} \cdot \left( {H_{vap} - H_{liq}} \right)}$

-   -   where dm/dt is the total mass flow rate, H is enthalpy and λ is         the fraction of liquid flashed to vapour in the valve.

The components described so far are located within a cryostat 20 comprising a number of shields to be described below.

The cold vapour leaving the second heat exchanger 6 passes up the cryostat 20 though another heat exchanger 10, absorbing heat from a gas cooled shield 7, which sits at a temperature at about 40K, and then through a final heat exchanger 11, absorbing heat from the second shield 8. The shields 7,8 of the cryostat 20 reduce the radiation heat load on the helium vessels 1 & 3, reducing total boil-off. Because the outer shield 8 sees the largest radiation load, it is common for it to have supplementary cooling from nitrogen boiling at atmospheric pressure (77K) in a vessel 8 a thermally connected to the shield 8. The entire vessel assembly is enclosed in an evacuated vessel 9 to reduce conduction and convection loss. The magnet 2 and inner vessels are typically suspended using a web of fibreglass rods (not shown) to reduce conduction heat load. A bore tube (not shown) at room temperature and pressure passes through the assembly and through the magnet bore 22 to allow samples to be placed inside the magnet 2.

After passing through a pump 13 located outside the cryostat 20 the helium gas is either vented to atmosphere and lost, or collected for later re-use (after re-liquefaction in a separate plant).

In the event of a magnet quench (a failure of superconducting state and release of stored magnetic energy as heat), the spring-closed pop-off valve 14 allows the boiling helium in the first vessel 1 to escape to the second vessel 3, and hence to atmosphere, before a dangerous over-pressure condition develops.

Whilst the system described above solves the refilling problem, it consumes a large quantity of helium. The global supply of helium is limited and prices are expected to rise significantly in the next decade. It is therefore desirable to reduce the quantity of helium used in a sub-cooled cryostat.

In accordance with the present invention, cooling apparatus comprises a cooling system defining a closed path around which a coolant flows, the system including a pump for causing coolant flow, a supply line extending from the pump to a cold location, positioned in a cryostat, in order to cool that location, and a return line extending from the cold location to the pump, the pump being located externally of the cryostat; a first heat exchanger positioned within the cryostat and linking the supply and return lines to allow heat exchange therebetween such that coolant flowing in the supply line is cooled by coolant flowing in the return line; and a refrigerator having a cooling stage within the cryostat and coupled to the supply line downstream of the first heat exchanger such that coolant reaching the first cooling stage has been precooled by the first heat exchanger.

We have developed a solution to the problem mentioned above by providing a cooling system defining a closed path around which the coolant, such as He, flows. This avoids the need to refill the cooling system and has the additional advantage that only small changes are required to be made to existing cooling systems in order to implement the invention. The solution involves utilizing a refrigerator having at least one cooling stage and assisting that cooling stage by including the first heat exchanger so as to precool the coolant before it reaches the first cooling stage. This reduces the power requirement of the first cooling stage to such an extent that conventional refrigerators such as pulse tube refrigerators, can be used. Typically, the cooling system includes a lambda point refrigerator located at the cold location while the cold location may be located within an auxiliary coolant containing vessel. Alternatively, an item to be cooled could be connected directly to the closed path of the cooling system.

Although in some cases a single, first heat exchanger is sufficient, particularly when cooling to higher temperatures, preferably the apparatus further comprises a second heat exchanger, located within the cryostat, and linking the supply and return lines such that coolant flowing in the supply line is cooled by coolant flowing in the return line, the second heat exchanger being upstream of the first heat exchanger with respect to coolant flow direction along the supply line.

The use of the second heat exchanger enables additional precooling to be achieved thus further producing the power requirements on the refrigerator. Of course, further heat exchangers could be provided if required.

In some cases, a single stage refrigerator can be used but in the preferred examples, the refrigerator has an additional cooling stage, warmer than the one coolant stage, the additional cooling stage being located within the cryostat and being coupled to the supply line to cool the supply line at a location upstream of the first heat exchanger.

In addition, or alternatively, the refrigerator has an additional cooling stage, warmer than the one cooling stage, the additional cooling stage being located within the cryostat and being coupled to a shield of the cryostat so as to cool the shield.

In the most preferred embodiment, the additional cooling stage cools both coolant in the supply line and the shield.

Where a second heat exchanger is provided, this is preferably located upstream of the one cooling stage of the refrigerator with respect to the direction of flow of coolant along the supply line.

Typically, the coolant which flows in the closed path of the cooling system comprises He although other coolants could be used depending upon the temperature required at the cold location. An alternative, for example is nitrogen.

The refrigerator is typically an electrically powered mechanical refrigerator such as a pulse tube refrigerator since this has minimum vibration problems. However, it will be appreciated that any cooler providing a low temperature cold stage and where coolant (such as ⁴He) is consumed, could be used. Therefore, alternatives to pulse tube refrigerators include Stirling, Gifford-McMahon, Joule-Thomson refrigerators, dilution refrigerators and so on.

As explained above, the cooling apparatus can be utilized to cool a variety of objects but it is particularly suited to the cooling of electrical conductors to their superconducting condition as required, for example, in NMR, MRI and ICR where superconducting magnets are required. In these cases, the magnets will define a bore, typically at room temperature and the surrounding vessels will be shaped to allow remote access to the bore.

An example of cooling apparatus according to the invention will now be described with reference to the accompanying drawings, in which:—

FIG. 1 is a schematic cross-section through a known cooling apparatus;

FIG. 2 illustrates the heat capacity of ⁴He near the lambda point; and,

FIG. 3 is a view similar to FIG. 1 but of an example of the invention.

In the following description, those components of the apparatus shown in FIG. 3 which are the same as those shown in FIG. 1 have been given the same reference numerals and will not be described again in detail.

In the FIG. 3 example, a closed cooling system is provided defined by a supply line 26 extending from the pump 13 via a pump filter 13 a into the cryostat 20 to the lambda refrigerator 4-6 and a return line 28 extending from the lambda refrigerator back to the pump 13. The supply line 26 opens into the second vessel 3 and is coupled to a second stage 16 of a two stage pulse tube refrigerator (PTR) 24. This second stage 16 recondenses helium vapour which boils in the vessel 3 and also condenses helium supplied along the supply line 26. It absorbs typically a few 10 s to 100 s mW of power.

Prior to reaching the second stage 16 of the PTR 24, the supply line extends through a “first” heat exchanger 17 which links the supply line 26 with the return line 28. This heat exchanger 17 allows the cold returning helium in the return line 28 to cool helium being supplied along the supply line 26 prior to reaching the first cooling stage 15. This reduces the cooling power required at the first cooling stage 15. This first heat exchanger 17 is particularly important because it keeps the cooling power requirement of the second stage 16 of the PTR 24 below about 1 W (the limit of current PTR technology at 4.2K).

A “second” heat exchanger 19 is provided upstream of the heat exchanger 17 with respect to the supply line. The heat exchanger 19 allows further heat exchange between the supply and return lines 26,28 so as to further precool the helium in the supply line.

A “third” heat exchanger 18 is provided coupled between the supply line 26 and the shield 8. The shield 8 is connected to a first stage 15 of the PTR 24 which is used to cool the outer shield 8 to about 40K, requiring about 30 W for a typical large NMR magnet system. In view of these connections, the first stage 15 of the PTR 24 cools both the shield 8 and helium in the supply line 26.

The heat exchangers 17 and 19 utilise the enthalpy of the cold gas leaving the lambda point refrigerator that, in the prior-art system, was used to cool the shields 7 and 8. The heat exchanger 18 adds a small heat load (a few watts) to the first stage 15 of the PTR 24.

Current PTR technology is limited to a cooling power in the second stage 16 of about 1 W at 4.2K. Without the heat exchanger 17 it would not be possible to re-condense all the helium boil-off from the lambda fridge in a typical state-of-the-art high-field NMR magnet cryostat. By exchanging the enthalpy of the warm gas with the fridge exhaust in exchanger 17 (and preferably also 19) the problem is solved.

It will be seen therefore that the invention provides a zero boil-off (ZBO) system consuming no helium in normal operation and thus providing significant advantages of no disruptive and costly refilling being required.

The effect of the “first” heat exchanger 17 can also be seen from the following analysis.

Assuming that the first stage 15 cools the vaporized helium to about 45K, then in the absence of the heat exchanger 17, the cooling power required by the second stage 16 must be: Q′=n′·(H _(T=45K) −H _(T=4.2))+n′·L=3.5·10⁻³·(932−87)+3.5·10⁻³·83=3+0.3≅3,3 Watt

-   -   Where L=83 J/Mol—helium latent heat,         -   n′=flow rate of helium,         -   H_(T)=Helium enthalpy

However, since the cooling power of the outgoing helium flow does not need to be utilized for cooling any radiation shields (unlike in the prior art) since the first stage 15 achieves this cooling, this cooling power can be used to pre-cool the incoming helium flow. The circulation of outgoing and return or incoming flows is the same and so pre-cooling of the return flow from 45K down to about 5 K can be achieved using the heat exchanger 17. As a result, the heating power required at the second stage 16 can be reduced to about 0.3-0.45 W. Such powers are readily available from commercially available pulse tube refrigerators.

As explained earlier, the system shown in the drawings can be used to cool a variety of items but particularly superconducting magnets which may be used in any conventional configuration such as MRI, NMR, and ICR. 

1. Cooling apparatus comprising a cooling system defining a closed path around which a coolant flows, the system including a pump for causing coolant flow, a supply line extending from the pump to a cold location, positioned in a cryostat, in order to cool that location, and a return line extending from the cold location to the pump, the pump being located externally of the cryostat; a first heat exchanger positioned within the cryostat and linking the supply and return lines to allow heat exchange therebetween such that coolant flowing in the supply line is cooled by coolant flowing in the return line; and a refrigerator having a cooling stage within the cryostat and coupled to the supply line downstream of the first heat exchanger such that coolant reaching the first cooling stage has been precooled by the first heat exchanger.
 2. Apparatus according to claim 1, wherein the cooling system includes a coolant containing vessel forming part of the supply line, the cooling stage of the refrigerator being located in the coolant containing vessel.
 3. Apparatus according to claim 2, wherein the coolant containing vessel is also connected to an auxiliary coolant containing vessel at the cold location whereby a portion of the coolant can flow from the coolant containing vessel to the auxiliary coolant containing vessel.
 4. Apparatus according to claim 1, wherein the cooling system includes a helium lambda-point refrigerator positioned at the cold location.
 5. Apparatus according to claim 1, further comprising a second heat exchanger, located within the cryostat, and linking the supply and return lines such that coolant flowing in the supply line is cooled by coolant flowing in the return line, the second heat exchanger being upstream of the first heat exchanger with respect to coolant flow direction along the supply line.
 6. Apparatus according to claim 1, wherein the refrigerator has an additional cooling stage, warmer than the one coolant stage, the additional cooling stage being located within the cryostat and being coupled to the supply line to cool the supply line at a location upstream of the first heat exchanger.
 7. Apparatus according to claim 1, wherein the refrigerator has an additional cooling stage, warmer than the one cooling stage, the additional cooling stage being located within the cryostat and being coupled to a shield of the cryostat so as to cool the shield.
 8. Apparatus according to claim 6, wherein the shield is coupled via a third heat exchanger to the supply line so that the additional cooling stage of the refrigerator cools both the shield and coolant in the supply line.
 9. Apparatus according to claim 8, wherein the second heat exchanger is located outside the shield.
 10. Apparatus according to claim 7, wherein the shield is also cooled by a second coolant contained within a second, coolant containing vessel attached to the shield.
 11. Apparatus according to claim 1, wherein the coolant in the cooling system comprises helium.
 12. Apparatus according to claim 1, wherein the refrigerator comprises a pulse tube cryocooler.
 13. Apparatus according to claim 1, wherein the refrigerator comprises a Stirling, Gifford-McMahon, Joule-Thomson or dilution refrigerator.
 14. Apparatus according to claim 1, wherein the one cooling stage of the refrigerator provides a temperature of about 4K.
 15. Apparatus according to claim 6, wherein the additional cooling stage of the refrigerator provides a temperature in the range 40-50K.
 16. Apparatus according to claim 1, further comprising a superconducting magnet located at the cold location.
 17. Apparatus according to claim 16, wherein the superconducting magnet defines a room temperature bore adapted to receive a sample.
 18. NMR or ICR apparatus including cooling apparatus according to claim 17; and a probe for insertion into the bore, the probe having means to support a sample. 